1. Field of the Invention
The present invention relates to a measuring circuit with a biosensor which utilizes ion sensitive field effect transistors (ISFETS).
2. Description of the Prior Art
Biosensors have been developed as a kind of chemical sensor for sensing biomaterials.
At present, biosensors are widely used in the fields of medical treatment, basic science and food engineering. As concerns for health are increased, the importance of these biosensors has also been increased.
Conventionally, for the measurement of the biomaterials such as urea, glucose and penicillin, etc., a biosensor utilizing an ion selective electrode or a gas sensing electrode has been widely and mainly used.
However, such type of biosensor has the disadvantages that it is very expensive, large in signal size, and slow in signal response as well as difficult to handle.
Since an ISFET is a semiconductor ion sensor and manufactured by an integrated circuit manufacturing process, it has numerous advantages such as: small size, easy standardization, rapid response, economic mass production, and relatively easy integration in a signal processing circuit.
The development potential of an ISFET biosensor is highly desirable and tremendously economical because as single metal such as platinum (Pt) wire can be used to substitute for a commercial reference electrode such as a reference FET (hereinafter referred to as REFET), which has been extremely difficult to be miniaturized and to be reduced in cost since the gate is not amenable to a sensing membrane even though it is capable of having the advantages as discussed above.
The conventional measuring circuit for an ISFET biosensor comprises a REFET which is not responsive to a specific biomaterial, an enzyme FET (hereinafter referred to as ENFET) which has an enzyme sensitive membrane on its gate and is responsive to the specific biomaterial, a sensed signal detecting circuit connected to the REFET and the ENFET, and a differential amplifier which amplifies differentially the output of the sensed signal detecting circuit.
However, such measuring circuit has many disadvantages because it has a complicated structure due to the numerous elements and it is difficult to miniaturize as an entire measuring system. Moreover, it is also difficult for the ISFETs and the measuring circuit to be integrated into one chip.